1. Field of Invention
The invention generally relates to controlling an optical amplifier. More particularly, the invention relates to circuits and apparatuses that control optical amplifiers having a fiber segment doped with an optically active material.
2. Description of Related Art
A variety of techniques have been employed to control optical amplifiers. One such conventional technique is the constant gain control loop. The conventional constant gain control loop is simple: measure the input and output powers and calculate the gain based on the input (Pin) and output (Pout) power measurements (measured gain=Pout/Pin). Changes are then made to the amplifier pump power such that the measured gain achieves the pre-stored or target gain value. In this way, it is possible to control the amplifier module to have a constant gain value regardless of changes to the input power level such as when optical WDM (wavelength division multiplexed) channel(s) are added or dropped.
In other words, traditional optical amplifier control works by comparing the measured optical gain (Pout/Pin) to a set (or target) gain, usually through digital means. This usually results in a series of discrete corrections to the pump laser(s) drive(s). As the input power approaches zero, however, severe problems arise because of the division by zero problem, see equation A) below.
Typical equations for the traditional optical amplifier constant gain control are:
A) Gmeas=Pout/Pin, where Gmeas is measured optical gain, Pin is optical input power, Pout is optical output power.
B) Gerr=Gmeas/Gset, where Gerr is gain error, Gset is the set gain. This equation can also be inverted, depending on following gain stages.
C) Pump laser drive(s) is the integral of Gmeas or Gerr.
While it is possible to compensate for this division by zero problem by switching to other equations when the input power (Pin) approaches zero, a discontinuity at the switchover point may occur which can cause noise and an inaccurate pump power level setting. Other functions may include undesirable nonlinearities.
Furthermore, such conventional constant gain control schemes use digital implementations because of the difficulty in measuring gain in analog circuitry. This difficulty partially stems from the very poor performance of analog circuits measuring voltage ratios. More significant is the dynamic range required for the application. For example, 10 mV in and 100 mV out would have to produce the same gain voltage (equivalent to a gain of 10) as 0.4V in and 4V out, a 40:1 dynamic range. As a further example consider a circuit having 10 mV in and 1V out: it would have to produce a voltage equivalent to a gain of 100, ten times higher voltage than the first example. Both issues apply for very low or zero inputs. One conventional solution is to narrow both the dynamic range of the acceptable inputs and the acceptable gains which is a serious compromise.
This invention does not require gain to be measured and does not measure or determine voltage ratios but instead compares the measured optical output power against the expected optical power, a function that is simple and fast to do in analog circuitry with no inherent non-linearities and which does not have the division by zero problem experienced by conventional constant gain control devices.